Polyurethanes and polyureas are known to be useful in curable compositions. They are typically prepared by reacting a polyisocyanate and an active hydrogen-containing chain extension agent. The polyurethane or polyurea is often capped with a polyol or reacted with excess polyol chain extension agent to provide it with a terminal hydroxyl group. This hydroxyl provides an active hydrogen that serves as a functional group to react with a curing agent such as an aminoplast resin. Other capping agents that will provide an active hydrogen functional group include amines, alcohol amines, and water.
It is often desireable for curable compositions to possess a variety of performance characteristics. These characteristics include good cohesiveness, strong adhesiveness to a variety of other materials, hardness, flexibility, scratch resistance, mar resistance, and the like. For coating compositions, especially for exterior coatings and most especially for clearcoats such as automotive clearcoats, resistance to attack by a variety of environmental elements, such as solvents, gasoline, water, salts, acids, heat, and UV radiation, are also desireable. While curable compositions based on hydroxy-capped polyurethanes or polyureas and melamine curing agents possess many desirable characteristics, they often do not provide an optimimum combination of the above performance charasterics as may be desired.